alanfleming

Here is the summary:

By the '90s microcomputers (computers based on single microprocessors) grew big enough to run Real OSes with memory protection etc. Most 20th century vendors did not have such things in their 1980/1990s OSes and lacked the ability to add them without destroying backwards compatibility.

The few OSes that could use this were niche things and either very expensive, and so died out, or they did next to nothing while also being almost impossible to use, and so nobody used them. 

Acorn, Commodore, and Atari died.

Apple nearly died. It had fired its boss, and he went off and built a new company with a super-hi-end all-singing-all-dancing computer. It flopped but the CIA bought loads of them and kept the company just about afloat.

Intel, and separately MS & IBM, tried to design their way out.

They all failed.

Even IBM struggled.

Meanwhile...

DEC inspired most of the industry but was even worse at marketing than Commodore. Unix, CP/M, DOS, NT, all were built on DEC designs and plans and ideas, or at least, on DEC hardware.

DEC had 1 brilliant team lead. He and his team built several of DEC's best-selling OSes. He said: we have half a dozen incompatible hardware lines and half a dozen incompatible OSes. A couple sell well but they're old. We should build a modern RISC chip and a modern modular OS on it that can emulate all the other OSes and converge on that.

Management said no, and spend billions on trying to scale the VAX minicomputer up into mainframe. It nearly died.

The team lead wanted out. MS headhunted him and the team.

MS handed him the skeleton of portable OS/2 (which it got from IBM in the divorce) and built that modular OS on it.

That's NT.

MS was rich enough to pay to develop it for a decade while it failed to sell much, while MS actually made money from selling upgrades to 16-bit Windows on top of DOS.

Eventually the hardware caught up. The new OS finally got halfway good enough 30Y ago last week. 

Apple, in dire straits, bought its old boss's company.

DEC survived. It salvaged the RISC CPU design, finished it, built it and it did well... and it knew MS had used its OS design and used that fact to screw a good deal out of MS, including porting that new OS to the DEC chip. It also gave the chips out to FOSS developers so it got the first ever Linux port.

Everyone else is dead.

Acorn is gone, but the Acorn OS is alive and it's FOSS now.

The ST and Commodore OSes are dead but have been reverse engineered and recreated by people who loved them.

All the Unix vendors are dead, but their OS was also re-created by volunteers, running on Windows boxes. It now runs the world... and most phones.